#!/usr/bin/env bash
# Creates "fixup!" commits for the staged hunks to appropriate
# ancestor changesets.

shopt -s lastpipe nullglob

boundary=$(git rev-parse "${1:-HEAD~5}" 2>/dev/null) || boundary=
fixupped_commit() {
	git blame --incremental -L "$2" "$boundary".. -- "$1" | IFS=\  read -r hash _
	[[ $hash != "$boundary" ]]
}

tmp=$(mktemp -d) || { echo 'Failed to create temporary directory'; exit 1; }
# shellcheck disable=SC2064
trap "rm -rf -- '$tmp'" EXIT

git diff-index --cached --unified=0 --no-prefix --ignore-submodules HEAD \
	| while :; do
	while IFS=\  read -r ext_header
		  case $ext_header in '') break 2 ;; 'index '*) false; esac
	do :; done
	IFS=\  read -r _ a; IFS=\  read -r _ b
	declare -A patched=()
	while IFS= read -r line &&
			[[ $line =~ ^@@\ -([0-9]+)(,([0-9]+))?' +'([0-9]+)(,([0-9]+))?\ @@ ]]; do
		l1=${BASH_REMATCH[1]} p=${BASH_REMATCH[3]:-1}
		l2=${BASH_REMATCH[4]} n=${BASH_REMATCH[6]:-1}
		mapfile -n $((p + n)) hunk
		{ [[ $a == "$b" ]] &&
			  fixupped_commit "$a" "$((p && l1>1 ? l1-1 : l1)),+$((p+2))"; } ||
			{ >&2 echo "$b: no parent found for $l2,+$n"; continue; }
		[[ ${patched[$hash]} ]] || cat << EOF >>"$tmp/$hash"
--- a/$a
+++ b/$b
EOF
		patched[$hash]=1
		{ echo "$line"; printf '%s' "${hunk[@]}"; } >>"$tmp/$hash"
	done
done

for patch in "$tmp"/*; do
	git read-tree --index-output="$tmp"/index HEAD
	GIT_INDEX_FILE="$tmp"/index git apply --cached --unidiff-zero "$patch"
	GIT_INDEX_FILE="$tmp"/index git commit --fixup="${patch#"$tmp"/}"
done